GTO auto trans shifting
 

When my 71 GTO with TH400 shifts from a standing start( casual speed, not hard acceleration, flat and level), the trans shifts from first approximately at 7 to 10 miles per hour. Barely gets moving before it shifts to second. This seems really fast to me or is it typical? I do not know what internals or shift kit or even what torque converter is installed with this trans.
Any suggestions/ input/ advice?
Thanks in advance.

You can somewhat tailor that with an adjustable vacuum modulator. Most of these cars over the years have already had one installed as that's what is readily available in the aftermarket.

If so, it's just a matter of removing the vacuum line on the modulator and with a very tiny screw driver you can turn the screw in (clockwise) to increase the shift points, and counterclockwise to lower them.

Increasing the shift points spreads them out a bit and will increase the firmness slightly as it now takes more vacuum to overtake the diaphragm. These adjustable units are also affected by elevation, as higher elevations will increase the shifts where engines make less vacuum, and lower elevations with more vacuum decrease the shifts. So depending on where you are located or where you drive the car will have an affect.

Formulajones;
Thank you. I will try that tomorrow. I had no idea it would be so easy!

Yes, it the modulator or if it's adjustable it's the setting.

Just keep in mind that low vacuum or no vacuum at the modulator will make the trans up shift way up at what the governor is calibrated to .

I thought that no vacuum would make the tranny shift very late? With no vacuum, the tranny thinks the engine is at WOT and not making much vacuum. Then it should shift late, when governor pressure (road speed) overcomes vacuum modulator pressure (engine load). With high vacuum, the tranny thinks the engine is lightly loaded and shifts early.

That's why turning the adj modulator clockwise to increase spring pressure on the diaphragm makes it shift a little later, more vacuum is then required to move the diaphragm. If that's not enough adjustment, you'll have to play with the governor springs to adjust shift MPH and shift spacing.

At least that's how it works on my TH350.

You are correct Chip.